This invention relates to conductive and optically transparent materials, and, more particularly, to conductive tin oxides and their production.
Films of tin oxide ordinarily are produced by the pyrolysis of inorganic, as well as organometallic compounds of tin, in an oxygen containing atmosphere. The properties of the films thus produced can be modified by the addition of small amounts of other elements such as phosphorus, arsenic, antimony, cadmium, indium, and fluorine. The resulting films or coatings are useful in many electronic and optical devices.
Previous processes for the production of such films have involved the spraying of tin-salt solutions on hot substrates. Examples are found in U.S. Pat. Nos. 2,617,745; 2,651,585 and 3,949,146. The spraying processes of these patents involve the possible inclusion of undesirable, toxic or corrosive constituents. They also produce such byproducts chlorine, hydrogen chloride and antimony halides. In addition the resulting products have inadequate physical properties.
Among the various dopants that are used for tin oxide modification, fluorine has been found to be a source of such desirable film properties as enhanced electrical conductivity and transparency. Fluorine is preferred over antimony which is metallic and toxic. The prior art has provided for the introduction of fluorine into tin oxide from hydrogen fluoride, elemental fluorine gas, or ammonium or alkali fluorides. These materials are all highly corrosive. Examples are found in U.S. Pat. Nos. 2,566,346 and 3,677,814. In the latter patent fluorine doped tin oxide is prepared from organotin fluorides. The latter fluorides are not only expensive to prepare, they are difficult to dissolve, and thus present difficulties in spraying. In addition the usual spray mixtures contain such corrosive components as amines and ammonium compounds. Although U.S. Pat. Nos. 4,265,974 and 4,146,657 teach the preparation of fluorine doped tin oxide by chemical vapor deposition, the fluorine source has been limited to either a fluorocarbon derivative of tin which has the fluorine atom in a alpha position relative to the tin, or to the use of fluorocarbon gases with an organotin vapor, which are limited to alpha-fluoroalkyls. The latter have the further objection of introducing secondary halogen atoms which can be undesirable in tin oxide.
Accordingly, it is an object of the invention to facilitate and simplify the production of conductive tin oxide and to realize tin oxide compositions which avoid many of the prior art objections.
Another object of the invention is to achieve the production of conductive tin oxide while avoiding the inclusion of undesirable, toxic or corrosive constitutents. A related object is to avoid the production of such byproducts as chlorine and hydrogen chloride.
Still another object of the invention is to improve the physical properties of tin oxide, in particular to increase both the conductivity and transparency of tin oxides. A related object is to achieve the modification of tin oxide by the avoidance of corrosive constituents. Another related object is to introduce fluorine as a dopant while avoiding the corrosive actions of elemental fluorine gas and ammonium or alkali fluorides.